Electrical Recording Thermometers for Clinical Work. 539 



37° C. for long periods. In the application of the resistance 

 method no thermostat was required, and it was easy to 

 obtain a tenfold larger deflexion with the same galvanometer. 

 The objections that have commonly been urged against 

 electrical resistance thermometers for this kind of work are 



(1) that resistance thermometers are more difficult to con- 

 struct and to insulate satisfactorily than thermocouples, and 



(2) that their indications are liable to be disturbed by the 

 heating effect of the current employed. These objections 

 undoubtedly exist, and have frequently proved fatal to the 

 employment of resistance thermometers : but they have 

 arisen chiefly from faulty application or construction, and 

 not from defects inherent in the method. Objection (1) is 

 readily surmounted by proper methods of construction, and 

 objection (2) by a proper consideration of the conditions of 

 sensitiveness. 



Conditions of Sensitiveness. 



2. The conditions of sensitiveness in measuring a resistance 

 by the Wheatstone bridge method have been discussed by 

 Maxwell ('Electricity and Magnetism/ vol. i. p. 437) and 

 Heaviside (Phil. Mag. Feb. 1873), whose results have 

 generally been quoted and applied to the problem under 

 consideration. They start with the assumption that the 

 battery power available is limited by the internal resistance 

 of the cells, and giA T e rules for obtaining the maximum 

 current through the galvanometer with this implied limit- 

 ation. The limitation of battery power was no doubt an 

 important consideration in many kinds of telegraph testing 

 thirty years ago, but it is rarely applicable in modern 

 laboratory practice, and never in dealing with electrical 

 resistance thermometers. In the majority of resistance mea- 

 surements, and more particularly in electrical thermometry, 

 the limiting condition is imposed by the heating effect of the 

 current on the resistance to be measured, and the resistances, 

 etc., should be chosen and arranged to give the greatest 

 sensitiveness for a given limiting value of the current through 

 the resistance to be measured. The resistance of the battery 

 circuit is quite immaterial provided that the battery can be 

 arranged to give the required limiting current. The problem 

 has been discussed from this point of view by Guye (Arch. 

 Sci. Phys. JS 7 at. Geneva, 1892), and by Schuster (Phil. Mag. 

 xxxix. p. 175, 1893), and in several of my own papers, but it 

 will facilitate discussion to reproduce here the investigation 

 itself in a simplified form. 



